How To Diognose Low Evaporator Heat Load, Fixed Orifice

Low evaporator heat loading occurs when there is not enough heat entering the refrigerant at the evaporator coil.

This condition can cause the evaporator coil to form a coating of frost or ice on it’s surface.  In extreme cases, superheat could fall to 0 degrees and liquid can flood back to the compressor.


  • Fixed orifice.  Systems running with low evaporator loading will run at pressures below factory required levels.  Abnormally low superheat and subcooling levels will be present when the evaporator coil lacks heat load.  The low heat load condition could cause the evaporator coil temperature to be very cold.  Ice or frost may form on the evaporator surface and suction line surface.  The compressor shell may sweat and the hot gas temperature will be cool.


  • Thaw out the evaporator coil if ice or frost are present.
  • Check airflow (350-450 CFM per ton)
  • Determine cause of low air flow, dirty filter, dirty evap coil, dirty blower, blower motor speed, restricted return duct.
  • Make sure thermostat setpoint is not too low.
  • Correct airflow problem.
  • Check for proper refrigerant charge.